Method for enriching raw salt

ABSTRACT

Process for enriching crude salt, in which the crude salt is ground and then cooled in a saturated aqueous sodium chloride solution, to a temperature below the anhydrous sodium chloride to sodium chloride dihydrate transition temperature, a stock of a powder comprising sodium chloride crystals is collected from the cooling step and the powder is subjected to particle-size fractionation from which a fine particle-size fraction and a coarse particle-size fraction comprising the enriched salt are collected.

The invention relates to a process for enriching crude salt.

It relates in particular to a process for obtaining, starting from crudesalt, salt enriched with sodium chloride, of purity sufficient to allowits use in an industrial process or as food.

Industrial salt and food salt are usually obtained starting from mineralsalt which is found in the natural solid state in the earth's crust(rock salt) or which is obtained by the evaporation of natural brackishwater, such as sea water or water from salt lakes. Crude mineral saltusually contains more than 80% (generally more than 90%) by weight ofsodium chloride. In crude mineral salt, the sodium chloride isaccompanied by impurities, especially calcium, magnesium and potassiumsalts (usually sulphates and/or chlorides) and iron oxides. Theseimpurities make crude mineral salt unsuitable, as it is, for mostindustrial applications or as food.

U.S. Pat. No. 3,655,333 provides a process making it possible to obtainhigh-purity sodium chloride starting from crude salt, in which the crudesalt is dispersed in a saturated solution of sodium chloride, theaqueous suspension thus obtained is cooled, below 0° C., in order tocrystallize the sodium chloride dihydrate, the crystals of sodiumchloride dihydrate are collected and washed with a pure aqueous solutionof sodium chloride. In this known process, the washing of the crystalsof sodium chloride dihydrate is a difficult, operation, theeffectiveness of which is not guaranteed. It involves the use of a pureaqueous solution of sodium chloride which is then thrown away, therebyconstituting a loss and increasing the cost of the sodium chlorideproduced.

The invention aims to remedy these drawbacks of the known processdescribed above, by providing a novel process making it possible toobtain, starting from crude salt, high-purity sodium chloride withoutrequiring a crystal-washing step.

The invention consequently relates to a process for enriching crudesalt, in which the crude salt is cooled in a saturated aqueous sodiumchloride solution to a temperature below the anhydrous sodium chlorideto sodium chloride dihydrate transition temperature and, after cooling,a stock of a powder comprising sodium chloride crystals is collected,the process being characterized in that the crude salt is ground beforeit is cooled and the powder is subjected to particle-size fractionationfrom which a fine particle-size fraction and a coarse particle-sizefraction comprising the enriched salt are collected.

The crude salt involved in the process according to the invention is asolid substance consisting of sodium chloride and of impurities,existing in the natural state (salt of mineral origin) or forming aresidue of an industrial process. Rock salt, sea salt obtained by solarevaporation of sea water, and salt coming from the evaporation of brineobtained by dissolving rock salt in situ in a deposit constituteexamples of crude salt of mineral origin. Another example of crude saltcoming within the scope of the invention is that produced as a residuein processes for scrubbing hydrogen-chloride-containing fumes by meansof sodium bicarbonate [SOLVAY (Société Anonyme), brochure Br1566a-B-1-0396]. The sodium chloride weight content of crude salt isgenerally greater than 80%, usually at least 90% (and frequently atleast 95%).

In the process according to the invention, the crude salt is ground at atemperature above the anhydrous sodium chloride to sodium chloridedihydrate transition temperature so that the sodium chloride present inthe salt collected from grinding is in the form of anhydrous sodiumchloride crystals. The grinding is generally carried out at ambienttemperature. The crude salt collected from grinding is cooled to belowthe anhydrous sodium chloride to sodium chloride dihydrate transitiontemperature. The purpose of cooling the salt is to recrystallize thesodium chloride into the form of sodium chloride dihydrate. The salt iscooled in a saturated aqueous sodium chloride solution. For thispurpose, various operating methods may be employed. According to a firstoperating method, the aqueous sodium chloride solution is used atambient temperature, the crude salt is introduced into it at ambienttemperature and the resulting aqueous suspension is cooled to below theaforementioned transition temperature.

According to another operating method, the crude salt being at ambienttemperature, the saturated aqueous sodium chloride solution is firstlycooled to a temperature below the said transition temperature and thenthe crude salt is introduced into the cold solution. The coolingtemperature of the ground crude salt is chosen so that the anhydroussodium chloride recrystallizes to sodium chloride dihydrate in thesaturated aqueous sodium chloride solution in a time which istechnically and economically acceptable. In practice, the ground crudesalt is cooled to a temperature which is more than 2° C. (preferably atleast 5° C.) below the aforementioned transition temperature.Consequently, the crude salt is generally cooled in the saturated sodiumchloride solution to a temperature below −2° C., at standard atmosphericpressure, the temperatures lying between −5 and −15° C. being especiallyrecommended.

After cooling, an aqueous stock of a powder comprising sodium chloridedihydrate crystals is collected. According to the invention, the saidpowder is subjected to particle-size fractionation. As a variant, beforethe powder is subjected to particle-size fractionation, the stock isheated to a temperature above the aforementioned transition temperaturein order to recrystallize the sodium chloride to the anhydrous state. Inthis alternative method of implementation of the process according tothe invention, the temperature to which the stock is heated is chosen sothat the sodium chloride dihydrate recrystallizes to anhydrous sodiumchloride in a time which is technically and economically acceptable. Inpractice, the stock is heated to a temperature which is more than 2° C.(preferably at least 5° C.) above the aforementioned transitiontemperature. Consequently, the stock is generally heated to atemperature above 2° C., at standard atmospheric pressure, thetemperatures lying between 5 and 15° C. being especially recommended.

In the present specification, the expression “powder comprising sodiumchloride crystals” denotes, equally well, either a powder comprisinganhydrous sodium chloride crystals or a powder comprising sodiumchloride dihydrate crystals, depending on whether or not theaforementioned variant of the process is carried out.

According to the invention, it is preferred to carry out the saidalternative method of implementation so that the powder subjected toparticle-size fractionation is a powder of anhydrous sodium chloridecrystals.

In the process according to the invention, the conditions employed ingrinding, on the one hand, and in cooling, on the other hand, are set sothat the particles of ground crude salt have a smaller mean diameterthan the mean diameter of the sodium chloride crystals in the powder ofthe stock collected from cooling.

The mean diameter of the crude salt particles (namely, sodium chloridecrystals) is defined by the mathematical equation$d = \frac{\sum{n_{i}d_{i}}}{\sum n_{i}}$

in which d denotes the mean diameter and n_(i) denotes the frequency byweight of the crude salt particles (namely, sodium chloride crystals) ofdiameter d_(i), the diameters d_(i) being measured by screeningaccording to the AFNOR standard.

In the process according to the invention, the purpose of theparticle-size fractionation is to split the particles of the stockpowder into two separate particle-size fractions (a coarse particle-sizefraction containing enriched salt and a fine particle-size fractioncontaining most of the crude-salt impurities). The particle-sizefractionation is set so that the cut-off diameter of the twoparticle-size fractions is approximately equal to the mean diameter ofthe sodium chloride crystals of the stock powder, the cut-off diameterbeing, by definition, the diameter of the holes in the standardizedscreen for particle-size measurement (according to the AFNOR standard)through which the entire fine particle-size fraction passes, the entirecoarse particle-size fraction being stopped by the screen. In order toimprove the degree of purity of the enriched salt, a cut-off diametergreater than the mean diameter of the sodium chloride crystals of thestock powder may be selected. All other things being equal, the greaterthe aforementioned cut-off diameter the higher is the level of purity ofthe enriched salt (and the higher is its sodium chloride content). Onthe other hand, the greater the difference between these two diametersthe greater is the sodium chloride loss in the fine particle-sizefraction. On the contrary, it may also be possible to select a cut-offdiameter slightly smaller than the mean diameter of the sodium chloridecrystals if it is desired to reduce the sodium chloride loss, thisreduction being, however, to the detriment of the purity of the enrichedsalt. The optimum cut-off diameter must consequently be determined ineach particular case according to a compromise between the degree ofpurity desired for the salt and the acceptable sodium chloride loss. Ingeneral, good results are obtained when the crude-salt particlescollected from grinding have a mean diameter of less than 200 μm,preferably less than 150 μm. Although in theory there is no lower limitto the mean diameter of the particles of ground crude salt, there is noadvantage in practice in grinding too finely, for economicconsiderations, and so as not to handicap the rest of the process. Inpractice, it is consequently advantageous for the crude salt collectedfrom grinding to have a mean diameter greater than 25 μm, preferablygreater than 50 μm. Mean diameters from 100 to 150 μm are recommended.Good results are obtained when the mean diameter of the crude saltcollected from grinding lies approximately between 100 and 150 μm, thecut-off diameter being between 80 and 150 μm, preferably at least equalto 100 μm.

The particle-size fractionation of the stock powder may be carried outby any suitable known means. According to a first method ofimplementation, the aqueous stock is filtered or decanted in order toextract the powder therefrom, this powder being subsequently dried andscreened. In another method of implementation, which is preferred, theparticle-size fractionation of the powder is carried out by elutriationof the aqueous stock. Elutriation is a well-known particle-size analysistechnique (Particle Size Measurement, Terence Allen, Chapman and Hall,London, 1974, pages 250-263).

In an advantageous method of implementing the process according to theinvention, the grinding of the crude salt is followed by particle-sizeseparation, by which the ground salt is split into a fine particle-sizefraction, which is collected and cooled in the aqueous sodium chloridesolution, and a coarse particle-size fraction which is discarded orrecycled for grinding. In this method of implementing the processaccording to the invention, it is desirable that the cut-off diameterfor particle-size fractionation of the powder comprising therecrystallized sodium chloride be not more than 20% (preferably not morethan 10%) less than the cut-off diameter for particle-size separation ofthe ground crude salt. Preferably, the cut-off diameter forparticle-size fractionation of the powder comprising recrystallizedsodium chloride is at least equal to the cut-off diameter forparticle-size separation of the ground crude salt. In general, it isrecommended to select a cut-off diameter of less than 200 μm forparticle-size separation of the ground salt. Good results are obtainedwhen the cut-off diameter for particle-size separation of the groundsalt is at most equal to 100 μm, the cut-off diameter for particle-sizefractionation of the powder being greater than 80 μm, preferably atleast equal to 100 μm.

In another advantageous method of implementing the process according tothe invention, the mother liquor collected from the stock afterseparating the powder is used to reconstitute the saturated aqueoussodium chloride solution used for cooling the ground crude salt. In thismethod of implementing the process according to the invention, it isdesirable to purify the mother liquor in order to remove therefrom atleast a fraction of the dissolved impurities. In practice, the fractionto be removed is approximately equal to the molar quantity of theimpurities present in the crude salt employed. In the particular case ofa crude salt of mineral origin (for example, rock salt), the motherliquor is normally contaminated with calcium, magnesium and sulphateions, coming from the main impurities of the crude salt (calciumsulphate and polyhalite) In this particular case, purification of themother liquor usually comprises adding sodium or calcium hydroxide tocrystallize and precipitate the magnesium as magnesium hydroxide andadding calcium chloride to crystallize and precipitate the sulphate ionsas gypsum. The respective quantities of sodium or calcium hydroxide andof calcium chloride are approximately equal to the quantities necessaryfor precipitating the calcium, magnesium and sulphate ions of the crudesalt employed.

As explained above, it is preferred, according to the invention, torecrystallize the sodium chloride dihydrate as anhydrous sodiumchloride, before subjecting the stock powder to particle-sizefractionation. In a particular variant of this preferred method ofimplementing the process according to the invention, the sodium chloridedihydrate is recrystallized as anhydrous sodium chloride in acrystallizer in which particle-size classification into twoparticle-size classes and removal of the finer of these two classes arecarried out simultaneously. In this particular method of implementingthe process according to the invention, the cut-off diameter of the twoparticle-size classes is approximately equal to the mean diameter of therecrystallized anhydrous sodium chloride crystals.

In the particular method of implementation just described, therecrystallization and simultaneous particle-size classification areadvantageously carried out in a fluidized bed of sodium chloridecrystals at a temperature below the aforementioned transitiontemperature (in the case of sodium chloride dihydrate crystallization)or at a temperature above the said transition temperature (in the caseof anhydrous sodium chloride crystallization). On passing through thefluidized bed, the sodium chloride in the stock recrystallizes as sodiumchloride dihydrate or as anhydrous sodium chloride, depending on thetemperature of the bed. The fluidization of the bed is set so that thestock powder splits up therein into two particle-size fractions, one ofwhich (the coarse fraction) is collected at the bottom of the bed andthe other of which (the fine particle-size fraction) is driven to thetop, out of the bed. In this method of implementing the processaccording to the invention, the crystals of the bed are fluidized by thestock blowing upwards through the bed. As a variant, in order to make iteasier to fluidize the bed, it is advantageous to dilute the stock in asaturated aqueous sodium chloride solution. In this variant of theinvention, the aqueous solution used for diluting the stock isadvantageously an aqueous solution which is separated from the fineparticle-size fraction collected at the top of the bed.

Special features and details of the invention will appear in the courseof the following description of the appended drawings, which arediagrams of plants for carrying out a few methods of implementing theprocess according to the invention.

FIG. 1 is a diagram of a plant for carrying out a first method ofimplementing the process according to the invention;

FIG. 2 is a diagram of a plant for carrying out a second method ofimplementing the process according to the invention;

FIG. 3 shows a detail of an alternative embodiment of the plant of FIG.2;

FIG. 4 shows a detail of another alternative embodiment of the plant ofFIG. 2.

In these figures, the same reference notations denote identicalelements.

In the plant illustrated in FIG. 1, crude salt 1 (for example, rocksalt) is introduced into a grinder 2, from which ground crude salt 3 ismoved. The ground crude salt 3 is sent to a screen 4 where it isseparated into two particle-size fractions 5 and 6. The screen 4 ischosen so that the cut-off diameter of the two particle-size fractionsis approximately equal to 100 μm. The coarse particle-size fraction 5 isrecycled back into the grinder 2 and the fine particle-size fraction 6is sent to a crystallization chamber 7. The crystallization chamber 7 ismoreover fed with a saturated aqueous sodium chloride solution 8 comingfrom a cooler 18, where it has been cooled to a temperature below theanhydrous sodium chloride to sodium chloride dihydrate transitiontemperature (for example, to a temperature lying between −5 and −10°C.). In the crystallization chamber 7, the solid salt 6 is dispersed inthe cold solution 8, this having the result of recrystallizing theanhydrous sodium chloride as sodium chloride dihydrate. An aqueous stock9 of a powder containing sodium chloride dihydrate crystals is removedfrom the crystallization chamber 7. The stock 9 is sent to a calibratedscreen 10, where the powder is separated into two particle-sizefractions 11 and 12, having a cut-off diameter of approximately 100 μm.The coarse particle-size fraction 11, retained on the screen, comprisessalt enriched with sodium chloride (as the dihydrate). This coarsefraction is sent to a heater 13 where it is heated to ambienttemperature, this having the effect of recrystallizing the sodiumchloride dihydrate as anhydrous sodium chloride. The enriched salt 14collected from the heater 13 is transferred to a drying plant, notillustrated.

The stock 12 collected from under the calibrated screen 10 contains thefine particle-size fraction of the powder, essentially consisting of theimpurities of the salt. It is sent to a filter 15 in order to separatetherefrom a solid fraction 16 and a mother liquor 17.

The mother liquor 17 is contaminated by dissolved impurities, especiallysodium sulphate and magnesium sulphate coming from the polyhalite of therock salt, and is sent to a purification chamber 36 where sodiumhydroxide 44 and calcium chloride 45 are added. Collected from thepurification chamber 36 are, on the one hand, a precipitate 46 ofmagnesium hydroxide and calcium sulphate dihydrate, which is removed,and, on the other hand, an aqueous sodium chloride solution 47 which issent into the aqueous solution 8, upstream of the cooler 18.

In the plant shown in FIG. 2, the cooler 18 is incorporated into thecrystallization chamber 7. The plant furthermore includes a mixingchamber 19 upstream of the crystallization chamber 7. The mixing chamber19 is fed with the fine fraction 6 of crude salt and with the saturatedaqueous sodium chloride solution 8, at ambient temperature. Collectedfrom the mixing chamber 19 is an approximately homogeneous suspension20, at ambient temperature, which is introduced, as such, into thecooling and crystallization chamber 7-18. In the latter, the aqueoussuspension 20 is cooled to below the anhydrous sodium chloride to sodiumchloride dihydrate transition temperature. Collected from the chamber7-18 is an aqueous stock 9 of sodium chloride dihydrate crystals, whichis introduced into a warming chamber 21, where it is heated to atemperature close to ambient temperature. The sodium chloride dihydrateof the stock recrystallizes in the chamber 21 as anhydrous sodiumchloride and a stock 22 of anhydrous sodium chloride crystals iscollected. The stock 22 is transferred to the top of an elutriationcolumn 23, of the levigation type, where it is subjected to the actionof a rising stream 24 of water or of approximately saturated aqueoussodium chloride solution. The velocity of the levigation stream 24 isset so as to fractionate the stock powder into two separateparticle-size fractions, the cut-off diameter of which is approximatelyequal to 100 μm. A portion 25 of the stock, containing the fineparticle-size fraction of the powder, is collected at the top of thecolumn 23 and the portion 26 of the stock, containing the coarseparticle-size fraction of the powder, is collected at the bottom of thecolumn. The portion 26 is treated over a filter 27, from which arecollected enriched salt 28, which is sent to a drying plant (notillustrated), and a mother liquor 29.

The portion 25 of the stock, containing the fine fraction, is treatedover the filter 15 in order to separate it into a solid fraction 16 anda mother liquor 17.

The mother liquors 17 and 29 are contaminated by dissolved impurities,especially sodium sulphate and magnesium sulphate coming from thepolyhalite of rock salt. They are collected in a purification chamber 36into which sodium hydroxide 44 and calcium chloride 45 are added.Collected from the purification chamber 36 are, on the one hand, amagnesium hydroxide and calcium sulphate dihydrate precipitate 46, whichis removed, and, on the other hand, an aqueous sodium chloride solution,a part 47 of which is sent into the aqueous solution 8 and another part38 of which is sent into the levigation stream 24.

In a variant (not illustrated) of the plant shown in FIG. 2, the motherliquors 17 and 29 are sent, as they are, into the mixing chamber 19,without passing through the purification chamber 36, and the reagents 44and 45 are introduced into the mixing chamber 19.

FIG. 3 shows a detail of a modified embodiment of the plant in FIG. 2.In this modified embodiment, the warming chamber 21 incorporates aparticle-size classifier which comprises, for example, a screen depicteddiagrammatically at 39. In this modified plant, a fraction 40 of themedium being crystallized is continuously removed, making it passthrough the screen 39, so as to leave in the chamber 21 the anhydroussodium chloride crystals which have recrystallized. Downstream of thescreen 39, the fraction 40 is treated over a filter 41 in order toseparate it from the fine solid particles 42, which are removed orrecycled into the grinder 2, and an aqueous solution 43 which isrecycled into the chamber 21. In the plant shown in FIG. 3, the screen39 is chosen so that it has approximately the same cut-off diameter asthat in the elutriation column 23. This plant according to the inventionthus has the particular feature of reducing the quantity of very fineimpurity particles in the chamber 21, which are liable to constituteparasitic-crystallization seeds for the anhydrous sodium chloride, andconsequently increases the degree of purity of the enriched salt 28.

FIG. 4 shows a detail of another modified embodiment of the plant ofFIG. 2. In this modified embodiment, the warming chamber 21 and theelutriation column 23 of FIG. 2 are replaced by apparatus which includes(FIG. 4) a crystallizer 30 of the fluidized-bed type. The crystallizer30 contains, above a fluidization mesh 31, a bed 32 of sodium chloridecrystals which is fluidized by a rising stream of a saturated aqueoussodium chloride solution 33. Crystallizers of the fluidized-bed type arewell-known in the art (GIVAUDON, Massot and BENSIMONT—“Précis de géniechimique” [Chemical engineering manual], Volume 1, 1960,Berger-Levrault, Nancy—pages 353 to 370). The crystal bed 32 in thecrystallizer 30 is maintained at ambient temperature. The stock 9 ofsodium chloride dihydrate crystals is introduced into the fluidizationstream 33 and driven with it into the bed 32. In the bed 32, the sodiumchloride dihydrate of the stock recrystallizes as anhydrous sodiumchloride. The velocity of the fluidization stream 33 is set so as tosubject the stock to particle-size separation by levigation so that thecoarse particles form a sediment in the bed and are collected at 34 fromthe bottom of the bed, while the fine particles are driven out of thebed and collected at the top of the crystallizer, as a dispersion 35 inthe fluidization solution. The particles 34 are treated in a dryer, notillustrated, and constitute the enriched salt. The aqueous dispersion 35is treated over the filter 15 from which are extracted a filter cake 16,which is removed, and a mother liquor 17 which is collected in thepurification chamber 36 where it is subjected to magnesium and sulphatepurification, as explained above. One part (47) of the purified motherliquor is recycled into the saturated solution 8 (FIG. 2) and anotherpart (38) is sent into the fluidization stream 33.

The following examples serve to illustrate the invention.

EXAMPLE 1

The process according to the invention has been applied to theenrichment of rock salt coming from the Borth mine in Germany. Thecontents of the main impurities (calcium, magnesium and sulphate ions)of the crude salt extracted from the mine are given below:

water 0.122 g/kg calcium (expressed 12.38 g/kg (dry salt) as CaSO₄)magnesium (expressed 1.44 g/kg (dry salt) as MgSO₄) SO₄ ions (expressed2.77 g/kg (dry salt) at Na₂SO₄, after removing the MgSO₄ and CaSO₄)other insoluble 0.68 g/kg (dry salt) impurities sodium chloride 982.73g/kg (dry salt)

The salt was ground and then treated over a screen, in order to removetherefrom, the fraction containing particle sizes greater than 100 μm.The fraction containing particle sizes less than 100 μm (the undersize)was recovered and subjected to the process according to the invention.For this purpose, part of the salt was dissolved in water so as to forma saturated aqueous sodium chloride solution. After filtering thesolution, in order to remove the insoluble matter from it, another partof the salt, of weight approximately equal to {fraction (1/9)} of theweight of the solution, was dispersed in the latter. The resultingaqueous stock was homogenized by stirring, at ambient temperature, andthen introduced into a thermostatically controlled chamber in which itwas gradually cooled down to approximately −10° C. and left there for atime sufficient for the anhydrous sodium chloride to be completelyrecrystallized as sodium chloride dihydrate. Next, the temperature inthe chamber was gradually raised to ambient temperature in order torecrystallize the sodium chloride dihydrate as anhydrous sodiumchloride. The stock was then extracted from the chamber and filtered.After drying, the filter cake was dried and then treated over a screenfor fractionating it into two particle-size classes, having a cut-offdiameter of 100 μm. The less-than-100 μm particle-size class was removedand the greater-than-100 μm particle-size class was collected. Thelatter represented 71.8% of the total weight of the cake (dry-matterweight). Its composition is given below:

water 0.165 g/kg calcium (expressed 1.8 g/kg (dry salt) as CaSO₄)magnesium (expressed 0.012 g/kg (dry salt) as MgSO₄) SO₄ ions 0.064 g/kg(dry salt) other insoluble 0.048 g/kg (dry salt) impurities sodiumchloride 998.08 g/kg (dry salt)

It may be seen that the process has appreciably enriched the salt, theweight content of sodium chloride of which exceeds 99.8%.

EXAMPLE 2

The test shown in Example 1 was repeated with rock salt bound to below100 μm, having the following respective contents of calcium, magnesiumand sulphate ions:

calcium 3.84 g/kg magnesium 0.32 g/kg sulphate 11.9 g/kg

The salt was subjected to the same treatment as in Example 1, thecooling having been carried out at a temperature of −7° C.(crystallization of sodium chloride dihydrate) and the stock having beenwarmed up to a temperature of +7° C. (crystallization of anhydroussodium chloride).

The salt, collected after the final treatment over the 100 μm screen andremoval of the fine particle-size fraction, was analyzed in terms of itscalcium, magnesium and SO₄ ion contents:

calcium 0.75 g/kg magnesium  0.0 g/kg sulphate  1.6 g/kg

EXAMPLE 3

In this example, the crystallization of anhydrous sodium chloride andthe subsequent particle-size classification were carried out in afluidized bed as described above with reference to FIG. 4. The othertest conditions were identical to those in Example 2. The purified saltwas analyzed in terms of its calcium and SO₄ ion contents:

calcium 0.21 g/kg sulphate  0.9 g/kg

What is claimed is:
 1. Process for enriching crude salt, in which thecrude salt is ground, the ground crude salt is cooled in a saturatedaqueous sodium chloride solution to a temperature, below the anhydroussodium chloride to sodium chloride dihydrate transition temperature, astock of a powder comprising sodium chloride crystals is collected fromthe solution, the powder is subjected to particle size fractionationfrom which a fine particle-size fraction and a coarse particle-sizefraction comprising the enriched salt are collected, and the sodiumchloride crystals in the powder have a higher mean diameter than themean diameter of the ground crude-salt particles.
 2. Process accordingto claim 1, comprising heating the stock to a temperature above theaforementioned transition temperature before the powder is subjected toparticle-size fractionation.
 3. Process according to claim 1, in whichthe saturated aqueous sodium chloride solution comprises a mother liquorseparated from the stock.
 4. Process according to claim 1, in which theparticle-size fractionation of the powder comprises a screening step oran elutriation step.
 5. Process according to claim 1, in which theparticle-size fractionation comprises an elutriation step which iscarried out in a fluidized bed of sodium chloride crystals at atemperature above the aforementioned transition temperature.
 6. Processaccording to claim 1, which comprises setting the particle-sizefractionation so that the fine particle-size fraction and the coarseparticle-size fraction have a cut-off diameter approximately equal tothe mean diameter of the crude-salt particles after grinding.
 7. Processaccording to claim 1, comprising subjecting the ground crude salt toparticle-size separation, collecting from said particle-size separationa fine particle-size fraction and a coarse particle-size fractioncooling said fine particle-size fraction in the aqueous sodium chloridesolution and removing said coarse particle-size fraction.
 8. Processaccording to claim 7, in which the particle-size fractionation of thepowder and the particle size separation of the ground crude salt haveeach a cut-off diameter such that the cut-off diameter for particle sizefractionation is no more than 20 percent less than the cut-off diameterfor particle-size separation.
 9. Process according to claim 8, in whichthe cut-off diameter for particle-size separation of the ground crudesalt is at most equal to 100 μm, the cut-off diameter for particle-sizefractionation of the powder being at least equal to 100 μm.